1. Field of the Invention
The present invention generally relates to heating devices, fuser devices, and image forming apparatuses, and particularly relates to a fuser device which is provided with a main power supply device for supplying power from an external power supply and an auxiliary power supply device capable of charging electricity based on the external power supply, these power supply devices serving as power supply means for supplying power to a heating unit.
2. Description of the Related Art
Image forming apparatuses such as copier machines form images on recording media such as paper sheets or overhead projector sheets. As consideration is given to the speed, image quality, and cost of an image forming process, an electrophotographic system is typically employed. The electrophotographic system forms a toner image on a recording medium, and fuses the formed toner image with the recording medium by applying heat and pressure. As a fusing system, a heat roller system is most commonly used from the safety point of view. The heat roller system includes a heating roller for providing heat by use of a heating member such as a halogen heater, and includes a mutually pressuring unit called a nip unit in which a pressuring roller is situated to oppose the heating roller and pressed against the heating roller. A recording medium on to which a toner image is transferred is passed through the nip unit, thereby experiencing heat and pressure, which fuse the toner image on the recording medium.
In recent years, out of consideration for environmental issues, there has been an ongoing effort to reduce power consumption in image forming apparatuses such as copier machines and printer apparatuses. In order to reduce power consumption in image forming apparatuses, an effective measure is to lower the temperature of the heating unit such as a fusing roller and fusing belt provided in the fuser device during the period in which no paper sheet is traveling in the image forming apparatus (e.g., during the standby period).
When the temperature of the heating unit is lowered during a no-paper-sheet-travel period, the heating unit needs to return to a certain temperature prior to the subsequent use of the fuser device. The user thus needs to wait in the meantime. Since it takes time before the apparatus becomes usable, convenience regarding the use of the apparatus is not satisfactory. In order to avoid such inconvenience, it is conceivable to shorten the time period in which the temperature of the heating unit is increased by providing an auxiliary power from an auxiliary power supply apparatus. In consideration of this, reducing the heat capacity of the fusing roller is desired.
The fusing roller comes in contact with a transfer member, and its heat is thus deprived of at the time of image formation. As a result, the temperature of the surface of the fusing roller drops rapidly despite the large heat capacity of the fusing roller, resulting in temperature below the minimum temperature necessary to achieve satisfactory fusing performance. In consideration of this, an auxiliary power supply apparatus that can be charged may be provided in an image forming apparatus, and supplies an electric power to the heater at the time of image formation, thereby preventing the temperature of the surface of the fusing roller from dropping.
Patent Document 1 discloses a fuser device provided with a main power supply device and an auxiliary power supply device, the fuser device having a heating unit comprised of a main heat generating body and an auxiliary heat generating body, the main heat generating body receiving an electric power from the main power supply device, and the auxiliary heat generating body receiving an electric power from the auxiliary power supply device having a capacitor. When heating of the heating unit starts, a large electric power is supplied from the main power supply device and from the auxiliary power supply device simultaneously, thereby raising the temperature of the heating unit to a desired temperature in a short time. During the standby period, the heating unit does not receive an electric power.
[Patent Document 1] Japanese Patent Application Publication No. 2002-184554
In the related-art fuser device as described above, the auxiliary power supply device is not charged by the external power supply at the time of image formation. There is thus a need to charge the auxiliary power supply device after the forming of one image before the forming of the next image. This results in an inconvenience in that a waiting period before the next image is formed is prolonged due to the time required for electric charge.
Further, during the forming of an image, the pressuring roller inside the fuser device is sufficiently heated. If the auxiliary power supply device is charged to its full capacity, and is then allowed to discharge for the purpose of heating, the heating unit ends up providing excessive heat to the transfer member and toner. This creates a risk of causing a failure such as the blocking of the toner and transfer member after the forming of the image.
Accordingly, there is a need for a heating device, a fuser device, and an image forming apparatus in which a time period between an image formation and the next image formation can be shortened, and, also, excessive heating of the heating unit can be prevented.